In a hydrofoil seacraft, the hull of the craft is lifted out of the water by means of foils which are carried on struts and usually pass through the water beneath the surface thereof. In passing through the water, and assuming that sufficient speed is attained, the foils create enough lift to raise the hull above the surface and, hence, eliminate the normal resistance encountered by a ship hull in passing through the water.
In the usual case, there are forward and aft foils both provided with control flaps similar to those used on aircraft, although in some cases the entire foils may be rotatable and used as control surfaces. The other essential control element is the rudder which pierces or is submerged beneath the surface of the water and may be either forward or aft of the craft, depending on its design. In most hydrofoils, the flaps or control surfaces are used primarily to cause the craft to ascend or descend and to control the craft about its pitch and roll axes; however, they can also be used in combination with the rudder to bank the ship about its roll axis during a turn. The flaps are also used to stabilize the craft during movement on water, so that pitching or rolling motions can be minimized by proper counterbalancing movement of the flaps. A control system of this type is disclosed and claimed, for example, in Stark et al U.S. Pat. No. 3,886,884 issued June 3, 1975 and assigned to the Assignee of the present application. The invention described herein is particularly useful in a system of the type described in the aforesaid patent; although its usefulness is not necessarily restricted to that particular system.
In the operation of hydrofoils such as that shown in the aforesaid U.S. Pat. No. 3,886,884, it has been found that good dynamic height and pitch control can be achieved, provided that the accumulation of manufacturing and assembly tolerance variations (depicted as "offsets") are relatively small. That is, if the manufacturing and assembly tolerances do not materially vary from the expected values, the system will function without any modification in accordance with theoretical parameters In actual practice, however, the tolerance variations are such that fixed biases must be applied to the hydrofoil control flap actuators in order to compensate for actual tolerance variations and achieve satisfactory height and pitch control. Determining the necessary values of the biases, however, is an involved and expensive procedure. This expense can be absorbed if the adjustments required are infrequent. However, in actual practice, parts replacement, wear and/or other causes result in the necessity for frequent adjustments with the result that the adjustment expense becomes a real burden.